The present invention relates to a distilling apparatus for distilling and purifying waste liquids such as sulfuric acid employed for chemical treatment in processes for producing semiconductors, etc.
As semiconductor devices such as LCD (Liquid Crystal Display) are produced in increasing numbers, huge amounts of solutions, such as of sulfuric acid, are employed. To achieve distillatory purification of waste liquids employed in production processes efficiently and accurately reduces production costs and contributes to environmental conservation.
In a process of producing semiconductor devices, for example, in a step of cleaning wafers, a solution prepared by adding about 2% by weight (wt %) of hydrogen peroxide to about 97 wt % of a commercially available sulfuric acid and heated to 80 to 120° C. is employed. When the waste liquid left after use of the solution is to be recovered, the added hydrogen peroxide is partly converted into water, and also, water migrates into the solution. Accordingly, the concentration of the sulfuric acid in the waste liquid drops to 80 to 90 wt %. Further, impurities such as silicon, iron, sodium and organic materials are contained in the waste liquid. The recovered waste liquid is sold or recycled by subjecting it to distillatory purification under atmospheric pressure. FIG. 1 shows an example of a prior art sulfuric acid distilling apparatus for distilling and purifying a sulfuric acid waste liquid.
As shown in FIG. 1, the sulfuric acid distilling apparatus is equipped with a concentrating column 101 and a purifying column 102. A contact type liquid level sensor 103 is disposed in the concentrating column 101. A valve 104 is opened and closed based on detection results of the liquid level sensor 103 such that a fixed amount of sulfuric acid waste liquid may be in the concentrating column 101. The waste liquid supplied to the concentrating column 101 is heated by a heater 105 to a predetermined temperature required to achieve distillation (e.g., 300° C. under the atmospheric pressure). The water content formed by distillation is condensed by a condenser 106, stored in a tank 107, and discharged by opening a valve 108. Water is supplied as a cooling medium to the condenser 106. The sulfuric acid thus concentrated is supplied to the purifying column 102.
The concentrated sulfuric acid is heated to a predetermined temperature (about 300° C. under atmospheric pressure) by a second heater 109 provided in the purifying column 102, and thus a purified sulfuric acid solution is formed. The purified sulfuric acid solution is passed through a condenser 110, stored in a tank 111 and can be discharged by opening a valve 112. Water is supplied as a cooling medium to the condenser 110. The residue in the purifying column 102 left, after purification, is condensed by a condenser 113 and stored in a tank 114. The residue in the tank 114 is discharged by opening a valve 115.
It is important to control the liquid level such that the amount of sulfuric acid waste liquid in the concentrating column 101 is constant in order to carry out correct control of distillation in the concentrating column 101. However, in the concentrating column 101, when the solution is heated, the hydrogen peroxide contained in the solution is vaporized to form foams, and also a large amount of hydrogen peroxide is decomposed to form oxygen foams. Such foaming greatly changes the level of the sulfuric acid waste liquid. Accordingly, the liquid level sensor 103 may erroneously detect the liquid level, making it is very difficult to control the amount of solution in the concentrating column 101.
Further, since the sulfuric acid in the concentrating column 101 is heated to about 300° C. or higher, related parts in the concentrating column 101 should have sufficient resistance to high temperatures of 300° C. or higher. Further, when the concentrating column 101 is heated to a high temperature, leakage at junctions and damage of the junctions can be induced by heat shock.
Japanese Unexamined Patent Publication No. 61-291407 discloses a process in which hydrogen peroxide contained in a sulfuric acid waste liquid is removed before the waste liquid is treated. According to this process, hydrogen peroxide is preliminarily removed from the sulfuric acid waste liquid by adding a sulfurous acid gas in an amount equal to or more than the equivalent amount of hydrogen peroxide to effect a chemical reaction. This process can prevent oxidation of a membrane for filtering the sulfuric acid waste liquid by hydrogen peroxide. However, the sulfurous acid does not entirely react with the hydrogen peroxide, and can form a reaction product of sulfuric acid and sulfurous acid. The reaction product is present in the sulfuric acid waste liquid as an impurity. As a result, the state of the sulfuric acid waste liquid is changed to lower the sulfuric acid purification accuracy.
The present invention provides a distilling apparatus which facilitates control of the amount of waste liquid.